EVIDENCE FOR SOLID-STATE DEFORMATION OF ~1.1 Ga "ANOROGENIC" GRANITES IN THE LLANO UPLIFT, TEXAS

Grain size reduction and crystal-plastic fabrics in two Town Mountain Granite (TMG) plutons in the central and southeastern Llano Uplift, suggest that some "anorogenic" TMG plutons were emplaced during the final stages of Grenville orogenesis.

TMG of the Grape Creek Pluton in the southeastern Uplift is cut by a NW-trending shear zone with a minimum exposed width of 400 m. Deformation in isotropic granite near the shear zone is localized within aplite dikes, which possess mylonitic to strongly foliated gneissic fabrics that parallel dike margins. All such dikes are parallel to the main shear zone. Within the shear zone, coarse-grained phenocrystic granite has been recrystallized to a protomylonite or augen gneiss that contains narrow zones of mylonite. The shear zone foliation strikes approximately 310 and dips 47-70° to the NE and contains a roughly down-dip stretching lineation. Asymmetric tails on recrystallized feldspar phenocrysts indicate reverse (top-to-the-southwest) sense of shear.

The interior of the Wolf Mountain Pluton shows a well-developed magmatic foliation defined by aligned K-feldspar phenocrysts. In many places this magmatic foliation is overprinted and slightly reoriented by solid-state deformation. This composite foliation is defined by alignment of K-feldspar and biotite and strikes approximately 320° and dips steeply east or west. A weak mineral stretching lineation plunges shallowly southeast. Feldspar porphyroclasts in sheared dikes parallel to the solid-state foliation indicate sinistral shearing. Deformation took place under conditions where granite behaved more competently than mafic enclaves within it, indicating deformation after granite solidification.

The solid-state foliation in both plutons is parallel to a late-stage Grenville country rock fabric that is axial planar to large open folds. The difference in deformation intensity between these plutons and the country rock, and the localization of deformation to NW-trending zones is consistent with emplacement during the waning stages of Grenville deformation.

Reed, Robert M. and Helper, Mark A. (1994) Evidence for solid-state deformation of ~1.1 Ga "anorogenic" granites in the Llano Uplift, Texas. Geological Society of America Abstracts with Programs, 26, 1, 25

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